In retrospect, it was obvious that it was no ordinary pick-up. Steven Toplass was waiting in the Andalucian sun for his lorry to be returned from a depot where it was to be packed with electronic goods. Finally it arrived, the cargo packaged and sealed by men that Toplass had never met. He phoned his fiancee in Stoke-on-Trent and told her he was coming home.

Within two hours the 38-year-old was in prison. Spanish police discovered 400kg of cannabis stacked in his trailer. More than 18 months on, Toplass is still in jail even though police documents suggest he is innocent. His case file reveals that even the officers who arrested him believe Toplass had no idea what was inside his trailer.

Nor is Toplass's plight unique. Lawyers believe up to 80 British lorry drivers are currently in Spanish and French jails, each an unwitting courier of the drug gangs who exploit the current boom in transcontinental lorry traffic.

Some of these drivers have never been charged. Some may never be. Many were caught travelling the main roads that wind from Andalucia through France and across the Channel to Europe's biggest drugs market. It is a famous trucking route; it is also the continent's so-called cannabis highway.

Those accused describe a trail of failed marriages, broken families and penury. David Stevenson lost his family while in jail. Single mother Karen Bland lost her home and contact with her three daughters. Toplass himself is close to breaking point. Letters sent daily from his Spanish cell reveal a man on the edge.

Now the lawyers charged with representing these disparate voices claim that their predicament raises searching questions over the nature of European justice and argue that they are neglected because working class truckers are not a political priority. In the case of Toplass, Stephen Jakobi, a human rights lawyer with more than 40 years' experience, says he has never seen a more 'compelling' case of an innocent man behind bars.

The story behind this suffering begins in the Rif Mountains of Morocco. In this rugged African kingdom lie vast plantations of cannabis. From the Rif peaks, you can make out the grey outline of mainland Europe. Beyond lies Andalucia and, slicing across its centre, the bustling A92 running east to Granada.

It was here that Toplass found himself on 6 February last year. He had been hired through a friend by Cameron Moir, owner of a Lancashire freight company for a one-off job. Once in Andalucia, Moir had told him that a man called 'Billy' would take his lorry and load the cargo of old computers himself. Toplass felt uneasy. For seven hours he waited, convinced the lorry had been stolen. Moir told him not to panic. When the vehicle appeared, its cargo had been vacuum-sealed and wrapped in thick plastic.

What no one knew at the time was that Toplass was being followed by British intelligence. Moir and his associates were suspected of operating a drug-smuggling ring. Transcripts of telephone calls made by Moir clearly reveal Toplass had no idea who he was working for. 'The driver (Toplass) is not friendly to our situation,' said Moir in one. In another, he said: 'We know he's (Toplass) not sympathetic to our plight.' Just 11 days after Toplass was arrested, Moir and his gang were arrested on charges of drug smuggling.

But Spanish authorities did not know that British intelligence had evidence indicating Toplass was innocent. Attempts to pass on that information were rejected by a Spanish judge. Last July Toplass was sentenced to three-and-a-half years in prison and fined £400,000.

Another two months passed before Toplass and his fiancee Diane Downes were told that the National Crime Intelligence Service (NCIS) had evidence suggesting the imprisoned truck driver was innocent. The only reason the existence of such evidence came to light was because police officers wanted Toplass to give evidence against Moir's gang.

Toplass, who did not attempt to bargain his position, proved a dream witness. Police intelligence said his testimony was key in jailing Moir's gang at Leeds Crown Court earlier this year. Moir received the same sentence as Toplass, whose health has deteriorated in the past year. His letters describe dramatic weight loss, cardiac problems and insomnia. Three teeth have fallen out, which he blames on being beaten by officers during his arrest.

'I feel I have been unfairly treated by a country and a judicial system that don't care as long as they have their conviction,' reads one letter. Diane Downs claims the 'sweetheart' she met at a local weekend karaoke night is edging towards a complete breakdown.

'He's desperate,' she told The Observer. 'He's saying everyone has forgotten him and that he has been left to rot.' Downs, 45, was recently forced to remortgage her small townhouse in Stoke-on-Trent to pay for the legal bills her job at a local school cannot cover. She dreams of another karaoke night and for the second year running will not celebrate Christmas.

Lorry driver Karen Bland would also rather forget festivities. A one-off job to France in October last year destroyed her life. French police found a stash of cannabis in the table legs of the furniture she had agreed to pick up for an employer who has since vanished.

She had accepted the job to pay for Christmas presents for her three daughters. Bland spent a year in prison but was never sent to court or charged. She has since been released on bail. No evidence has been found linking her to drugs. Sabine Zanker, a European lawyer for Fair Trials Abroad, is convinced Bland is one of scores of innocent truckers.

'The fact she was released on bail implies they have nothing on her,' Zanker said. During her internment, Bland lost her family home in Hampshire while her daughters were forced to live with their grandparents. 'She's lost it all: her house, her everything,' said her grandfather.

David Stevenson, 48, is another victim of a French and Spanish system which favours rapid, presumptive judgments. Three years ago, Stevenson picked up a pre-loaded sealed trailer in northern France that police subsequently found contained cannabis. Two days later Stevenson was sentenced to two years in a Dunkirk cell with 20 inmates. His appeal simply encouraged the French authorities to add another year to his sentence. His lorry's tachograph suggested he must have loaded 250kg of cannabis on five pallets in less than 20 minutes on a busy garage forecourt. CCTV footage from the garage was never sought by the French authorities.

Again, the personal cost was enormous. 'I survived, but everything else didn't.' He now lives in Hastings, Sussex, and admits to feeling embittered each time he gazes across the channel to the country 22 miles south.

The transcripts of the telephone calls intercepted by police as they sought to obtain evidence against Moir offer a rare glimpse into the gangs who control cannabis. Their banter is a parody of macho threats and boasts. They talk of geezers, 'bags of sand' for a thousand pounds and 'timber' as a codeword for wood or cannabis. But it is the references to the 'chauffeur' or Toplass that prove most interesting. Clearly, there is an awareness that those like Toplass carry all the risk. Spanish and French authorities do not have to notify the driver's company, the British authorities or next of kin before charging.

By contrast, airline pilots and ships' captains are not automatically guilty when drugs are found in their charge. So high are the stakes that Jakobi likens their livelihood to a game of 'Russian roulette'. Odds, he believes, mean more like Toplass will follow.